Routing in Ad-Hoc Networks

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Contents: Chapter/Unit-2 2.1 2.2 2.3

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Introduction Topology-Based Verses Position-Based Approaches Topology-Based Routing Protocols 2.3.1 Proactive Routing Approach 2.3.2 Reactive Routing Approach 2.3.3 Hybrid Routing Approach 2.3.4 Comparisons Position-Based Routing Protocols 2.4.1 Principles and Issues 2.4.2 Location services 2.4.3 Forwarding Strategies 2.4.4 Comparison Other Routing Protocols 2.5.1 Signal Stability Routing 2.5.2 Power Aware Routing 2.5.3 Associative Routing 2.5.4 QoS Routing

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Introduction A MANET environment, illustrated in Figure 2.1(a), is characterized by energy-limited nodes (Mobile Hosts), bandwidth-constrained, Variable-capacity wireless links and dynamic topology, leading to frequent and unpredictable connectivity changes. For example, In Figure 2.1(a) that node S uses node B to communicate with node D. However, as nodes in a MANET are mobile, it may so happen that the route from node S to node D changes while in use, and now traverses nodes A and B as in Figure 2.1(b). Therefore, traditional link-state and distance vector routing are not effective in this environment

Numerous MANET routing protocols have been proposed, both under and outside the umbrella of the IETF MANET working group. We use the term MH and node interchangeably throughout the text. Routing in a MANET depends on many factors including topology, selection of routers, and location of request initiator, and specific underlying characteristics that could serve as a heuristic in finding the path quickly and efficiently. One of the major challenges in designing a routing protocol for MANETs is that a node at least needs to know the reachability information to its neighbors for determining a packet route, while the network topology can change quite often in a MANET.

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Routing in Ad-Hoc Networks Furthermore, as the number of network nodes can be large, finding route to a destination also requires frequent exchange of routing control information among the nodes. Thus, the amount of update traffic can be substantial, and it is even higher when nodes with increased mobility are present. The MHs can impact route maintenance overhead of routing algorithms in such a way that no bandwidth might be left for the transmission of data packets.

2.2 Topology-Based versus Position-Based Approaches Routing over ad hoc networks can be broadly classified as topologybased or position-based approaches. Topology-based routing protocols depend on the information about existing links in the network and utilize them to carry out the task of packet forwarding. They can be further subdivided as being Proactive (or tabledriven), Reactive (or on-demand), or Hybrid protocols. Proactive algorithms employ classical routing strategies such as distance-vector or link-state routing and any changes in the link connections are updated periodically throughout the network. They mandate that MHs in a MANET should keep track of routes to all possible destinations so that when a packet needs to be forwarded, the known route can be used immediately. Proactive protocols have the advantage that a node experiences minimal delay whenever a route is needed as a route is immediately obtained from the routing table. However, proactive protocols may not always be appropriate in MANETs with high mobility. This may cause continuous use of a substantial fraction of the network capacity so that the routing information could be kept current. In addition, the quality of channels may change with time due to the shadowing and fast fading and may not be good to use even if there is no mobility Reactive protocols employ a lazy approach whereby nodes only discover routes to destinations on-demand. i.e they find a route to a destination only when needed. They often consume much less bandwidth, but the delay in determining a route can be substantially large. Another disadvantage is that, even though route maintenance is limited to routes currently in use, it may still generate a significant amount of network control traffic when the topology of the network changes frequently. Lastly, packets can route to the destination are likely to be lost if the route in use changes.

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Routing in Ad-Hoc Networks Hybrid protocols combine local proactive and global reactive routing in order to achieve a higher level of efficiency and scalability. For example, a proactive scheme may be used for close by MHs only, while routes to distant nodes are found using reactive mode. Usually, but not always, hybrid protocols may be associated with some sort of hierarchy which can either be based on the neighbors of a node or on logical partitions of the network. The major limitation of hybrid schemes combining both strategies is that it still needs to maintain at least those paths that are currently in use. This limits the amount of topological changes that can be tolerated within a given time span. Position-based routing algorithms overcome the limitations of topology-based routing by relying on the availability of additional knowledge. These require that the physical location information of the nodes be known. Each or some of the MHs determine their own position using Global Positioning System (GPS) technique. The sender normally uses a location service to determine the position of the destination node, and to incorporate it in the packet destination address field. Here, the routing process at each node is based on the destination's location available in the packet and the location of the forwarding node's neighbors. Position-based routing does not require establishment or maintenance of routes, but this usually comes at the expense of an extra hardware. As a further enhancement, position-based routing supports the delivery of packets to all nodes in a given geographical region in a natural way, and this is called Geocasting which is discussed in the next chapter.

2.3 Topology-Based Routing Protocols 2.3.1 Proactive Routing Approach 2.3.1.1 Destination-Sequenced Distance-Vector Protocol

The destination-sequenced distance-vector (DSDV) is a proactive hop-by-hop distance vector routing protocol, requiring each node to broadcast routing updates periodically. Here, every MH in the network maintains a routing table for all possible destinations within the network and the number of hops to each destination. Each entry is marked with a sequence number assigned by the destination MH. The sequence numbers enable the MHs to distinguish stale routes from new ones, thereby avoiding the formation of routing loops. Routing table updates are periodically transmitted throughout the network in order to maintain consistency in the tables.

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Routing in Ad-Hoc Networks To alleviate potentially large network update traffic, two possible types of packets can be employed: Full Dumps or Small Increment packets. A full dump type of packet carries all available routing information and can require multiple network protocol data units (NPDUs). These packets are transmitted less frequently during periods of occasional movements. Smaller incremental packets are used to relay only the information that has changed since the last full dump Each of these broadcasts should fit into a standard-size NPDU, thereby decreasing the amount of traffic generated. The MHs maintain an additional table where they store the data sent in the incremental routing information packets. New route broadcasts contain the address of the destination, the number of hops to reach the destination, the sequence number of the information received regarding the destination, as well as a new sequence number unique to the broadcast. The route labeled with the most recent sequence number is always used. In the event that two updates have the same sequence number, the route with the smaller metric is used in order to optimize the path. MHs also keep track of settling time of the routes, or the weighted average time that routes to a destination could fluctuate before the route with the best metric is received. By delaying the broadcast of a routing update by the length of the settling time, MHs can reduce network traffic. Note that if each MH in the network advertises a monotonically increasing sequence number for itself, it may imply that the route just got broken. For example, MH B in Figure 2.1 decides that its route to a destination D is broken, it advertises the route to D with an infinite metric. This results in any node A, which is currently routing packets through B, to incorporate the infinite-metric route into its routing table until node A hears a route to D with a higher sequence number. 2.3.1.2 The Wireless Routing Protocol

The Wireless Routing Protocol (WRP) is a table-driven protocol with the goal of maintaining routing information among all nodes in the network. Each node in the network is responsible for maintaining four tables: Distance table, Routing table, Link-cost table, and the Message Retransmission List (MRL) table. Each entry of the MRL contains the sequence number of the update message, a re-transmission counter, an acknowledgment-required lag vector with one entry per neighbor, and a list of updates sent in the update message. The MRL records which updates in an update message ought to be retransmitted and neighbors need to acknowledge the K. V. Pradeep

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retransmission. MHs keep each other informed of all link changes through the use of update messages. An update message is sent only between the neighboring MHs and contains a list of updates (the destination, the distance to the destination, and the predecessor of the destination), as well as a list of responses indicating which MHs should acknowledge (ACK) the update. After processing updates from neighbors or detecting a change in a link, mobile nodes send update messages to a neighbor. Similarly, any new paths are relayed back to the original MHs so that they can update their tables accordingly. MHs learn about the existence of their neighbors from the receipt of acknowledgments and other messages. If a MH does not send any message for a specified time period, it must send a hello message to ensure connectivity. Otherwise, the lack of messages from the MH indicates the failure of that link and this may cause a false alarm. Whenever a MH receives a hello message from a new MH, it adds this new MH to its routing table and sends a copy of its routing table information to this new MH. Part of the novelty of WRP stems from the way in which it achieves freedom from loops. In WRP, nodes communicate the distance and second-to-last hop information for each destination in the network. WRP belongs to the class of path-finding algorithms with an important exception that it avoids the "count-to-infinity" problem by forcing each node to perform consistency checks on predecessor information reported by all its neighbors. This ultimately (although not instantaneously) eliminates looping situations and provides faster route convergence if and when a link failure occurs. 2.3.1.3 The Topology Broadcast based on Reverse Path Forwarding Protocol

The Topology Broadcast based on Reverse Path Forwarding (TBRPF) protocol considers the problem of broadcasting topology information (including link costs and up/down status) to all nodes of a communication network. This information, together with a path selection algorithm, can be used by each node to compute preferred paths to all destinations, i.e., to perform routing based on link states. Most link-state routing protocols, including the Open Shortest Path First (OSPF), are based on flooding. In these protocols, each link-state update is sent on every link of the network. Although flooding is useful in networks with high bandwidth links, it can consume a significant percentage of link bandwidth in MANETs where the network contains links with relatively low bandwidth. The communication cost of broadcasting topology information can be reduced if the updates are sent along spanning trees. However, there is additional K. V. Pradeep

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Routing in Ad-Hoc Networks communication cost for maintaining these trees. The main concern here is whether the total communication cost is significantly less as compared to this additional cost. The TBRPF protocol is based on the extended reverse-path forwarding (ERPF) algorithm in which messages generated by a given source are broadcast in the reverse direction along the directed spanning tree formed by the shortest paths from all nodes to the source. ERPF assumes the use of an underlying routing algorithm by each node i in selecting the next node pi(v) along the shortest path to each destination (or broadcast source) v. The node pi(v) then becomes the parent of i on the broadcast tree rooted at source v. Each node informs its parent of this selection, so that each parent becomes aware of its children for each source. A node i receiving a broadcast message originating from source v from its parent pi(v) forwards the message to its children for source v (if it has children). ERPF is not reliable when the shortest paths can change due to the dynamic topology. In fact, since ERPF is not reliable, the underlying routing algorithm should not depend on ERPF for topology broadcast. TBRPF combines the concept of ERPF with the use of sequence numbers to achieve reliability, and the computation of minimum-hop paths based on the topology information received along the broadcast tree rooted at the source of the information. Since minimum-hop paths are computed, each source node broadcasts link-state updates for its outgoing links along a minimum-hop tree rooted at the source. Therefore, a separate broadcast tree is created for each source. The use of minimum-hop trees instead of shortest-path trees (based on link costs) results in less frequent changes in the broadcast trees and therefore less communication cost to maintain the trees. TBRPF has the following chicken-egg paradox: it computes the paths for the broadcast trees based on the information received along the trees themselves. Thus, the correctness of TBRPF is not obvious. However, it is shown in that every MH knows the correct topology in finite time using TBRPF, if no topology changes occur for some time. TBRPF is a simple, practical protocol that generates less update/control traffic than flooding and is therefore especially useful in networks that have frequent topology changes and have limited bandwidth.

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2.3.1.4 The Optimized Link State Routing Protocol

The Optimized Link State Routing (OLSR) protocol is a proactive protocol based on the link state algorithm. In a pure link state protocol, all the links with neighboring nodes are declared and are flooded in the entire network. OLSR protocol is an optimization of a pure link state protocol for MANETs. First, it reduces the size of control packets: instead of all links, it declares only a subset of links amongst its neighbors which serves as its multipoint relay selectors (described next). Secondly, it minimizes flooding of this control traffic by using only the selected nodes, called multipoint relays, in diffusing its messages throughout the network. Apart from normal periodic control messages, the protocol does not generate extra control traffic in response to link failures or additions. The protocol keeps the routes for all the destinations in the network, hence it is beneficial for the traffic patterns with a large subset of MHs are communicating with each other, and the pairs are also changing with time. The protocol is particularly suitable for large and dense networks, as the optimization done using the multipoint relays works well in this context. OLSR is designed to work in a completely distributed manner and thus does not depend upon any central entity. It does not require a reliable transmission for its control messages: each node sends its control messages periodically, and can therefore sustain a loss of some packets from time to time, which happens very often in radio networks due to collisions or other transmission problems. In addition, OLSR does not need an in-order delivery of its messages: each control message contains a sequence number of most recent information therefore reordering can be done at the receiving end. OLSR protocol performs hop-by-hop routing, i.e., each node uses its most recent information to route a packet. Therefore, when a node is moving, its packets can be successfully delivered to it, if its speed is such that its movement could at least be followed in its neighborhood. 2.3.1.4.1 Multipoint Relays

The idea of multipoint relays is to minimize the flooding of broadcast packets in the network by reducing duplicate retransmissions in the same region. Each MH in the network selects a set of neighboring MHs, to retransmit its packets and is called the multipoint relays (MPRs) of that node. The neighbors of any node N which are not in its MPR set receive the packet but do not retransmit it. Every broadcast message coming from these MPR Selectors of a node is assumed to be retransmitted by that node. This set can K. V. Pradeep

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Routing in Ad-Hoc Networks change over time and is indicated by the selector nodes in their hello messages. Each node selects its multipoint relay set MPR among its one hop neighbors in such a manner that the set covers (in terms of radio range) all the nodes that are two hops away. The smaller is the multipoint relay set, the more optimal is the routing protocol. Figure 2.2 shows the multipoint relay selection around MH N. Multipoint relays are selected among the one-hop neighbors with a bidirectional link. Therefore, selecting the route through multipoint relays automatically avoids the problems associated with data packet transfer on unidirectional links.

2.3.1.5 The Source Tree Adaptive Routing Protocol

The Source Tree Adaptive Routing (STAR) protocol does not use periodic messages to update its neighbors. STAR is an attempt to create the same routing performance as the other proactive protocols and still be equal or better on bandwidth efficiency. To be able to do this, on demand route optimization has been put aside and the routes are allowed to be non-optimal to save bandwidth. However, STAR depends on an underlying protocol which must reliably keep track of the neighboring MHs. This could be implemented with periodic messages, but is not required. In addition to this, the link layer must provide reliable broadcasting, or else this feature will have to be implemented into STAR with an extra routing rule.

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2.3.2 Reactive Routing Approach 2.3.2.1 Dynamic Source Routing The Dynamic Source Routing (DSR) algorithm is an innovative approach to routing in a MANET in which nodes communicate along paths stored in source routes carried by the data packets. It is referred to as on demand protocol. In DSR, MHs maintain route caches that contain the source routes which the MH is aware of. Entries in the route cache are continually updated as new routes are learned. The protocol consists of two major phases: route discovery and route maintenance. When a MH has a packet to send to some destination, it first consults its route cache to determine whether it already has a route to the destination. If it has a route to the destination, it will use this route to send the packet. On the other hand, if the MH does not have such an unexpired route, it initiates route discovery by broadcasting a route request packet. This route request contains the address of the destination, along with the source MH's address and a unique identification number. Each node receiving the packet checks whether it knows of a route to the destination. If it does not, it adds its own address to the route record of the packet and then forwards the packet along its outgoing links. To limit the number of route requests propagated on the outgoing links of a MH, a MH only forwards the route request if it has not yet seen the request and if the mobile MH's address does not already appear in the route record. A route reply is generated when the route request reaches either the destination itself, or an intermediate node that in its route cache contains an unexpired route to the destination. By the time the packet reaches either the destination or such an intermediate node, it contains a route record with the sequence of hops taken. Figure 2.3 (a) illustrates the formation of the route as the route request propagates through the network. If the node generating the route reply is the destination, it places the route record contained in the route request into the route reply. If the responding node is an intermediate node, it appends its cached route to the route record and then generates the route reply. To return the route reply, the responding node must have a route to the initiator. If it has a route to the initiator in its route cache, it may use that route. Otherwise, if symmetric links (defined in Chapter 1) are supported, the node may reverse the route in the route record. If symmetric links are not supported, the node may initiate its own route discovery and piggyback the route reply on the new route request. Figure 2.3 (b) shows the transmission of route record back to the source node. K. V. Pradeep

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Routing in Ad-Hoc Networks Route maintenance is accomplished through the use of route error packets and acknowledgments. Route error packets are generated at a node when the data link layer encounters a fatal transmission problem. When a route error packet is received, the hop in error is removed from the node's route cache and all routes containing the hop are truncated at that point. In addition to route error messages, acknowledgments are used to verify the correct operation of the route links. These include passive acknowledgments, where a MH is able to hear the next hop forwarding the packet along the route.

DSR also supports multi-path in its design as a built-in feature with no need for extra add-ons. This comes in very handy when a route fails, another valid route can be obtained from the route cache if one exists. In other words, the route cache itself possesses the multi-path capability by allowing the storage of more than one route to a destination.

2.3.2.2 The Ad Hoc On-Demand Distance Vector Protocol The Ad Hoc On-Demand Distance Vector (AODV) routing protocol is basically a combination of DSDV and DSR. It borrows the basic on-demand mechanism of Route Discovery and Route Maintenance from DSR, plus the use of hop-by-hop routing, sequence numbers, and periodic beacons from DSDV. AODV minimizes the number of required broadcasts by creating routes only on-demand basis as opposed to maintaining a complete list of routes as in the DSDV algorithm. Authors of AODV classify it as a pure on-demand route acquisition system since MHs that are not on a selected path, do not maintain routing information or participate in routing table exchanges. It supports only symmetric links with two different phases: • Route Discovery, Route Maintenance; and • Data forwarding. K. V. Pradeep

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Routing in Ad-Hoc Networks When a source MH desires to send a message and does not already have a valid route to the destination, it initiates a path discovery process to locate the corresponding MH. It broadcasts a route request (RREQ) packet to its neighbors, which then forwards the request to their neighbors, and so on, until either the destination or an intermediate MH with a "fresh enough" route to the destination is reached. Figure 2.4(a) illustrates the propagation of the broadcast RREQs across the network. AODV utilizes destination sequence numbers to ensure all routes are loop-free and contain the most recent route information. Each node maintains its own sequence number, as well as a broadcast ID. The broadcast ID is incremented for every RREQ the node initiates, and together with the node's IP address, uniquely identifies an RREQ. Along with the node's sequence number and the broadcast ID, the RREQ includes the most recent sequence number it has for the destination. Intermediate nodes can reply to the RREQ only if they have a route to the destination whose corresponding destination sequence number is greater than or equal to that contained in the RREQ. During the process of forwarding the RREQ, intermediate nodes record in their route tables the address of the neighbor from which the first copy of the broadcast packet was received, thereby establishing a reverse path. If additional copies of the same RREQ are later received, they are discarded. Once the RREQ reaches the destination or an intermediate node with a fresh enough route, the destination/intermediate node responds by unicasting a route reply (RREP) packet back to the neighbor from which it first received the RREQ (Figure 2.4(b)). As the RREP is routed back along the reverse path, nodes along this path set up forward route entries in their route tables that point to the node from which the RREP came. Associated with each route entry is a route timer which causes the deletion of the entry if it is not used within the specified lifetime. Because the RREP is forwarded along the path established by the RREQ, AODV only supports the use of symmetric links.

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Routing in Ad-Hoc Networks Routes are maintained as follows. If a source node moves, it is able to reinitiate the route discovery protocol to find a new route to the destination. If a node along the route moves, its upstream neighbor notices the move and propagates a link failure notification message (an RREP with infinite metric) to each of its active upstream neighbors to inform them of the breakage of that part of the route. These nodes in turn propagate the link failure notification to their upstream neighbors, and so on until the source node is reached. The source node may then choose to re-initiate route discovery for that destination if a route is still desired. An important aspect of the protocol is the use of hello messages as periodic local broadcasts to inform each MH in its neighborhood. Hello messages can be used to maintain the local connectivity in the form of beacon signals. However, the use of hello messages may not be required at all times. Nodes listen for re-transmission of data packets to ensure that the next hop is still within reach. If such a retransmission is not heard, the node may use techniques to determine whether the next hop is within its communication range. The hello messages may also list other nodes from which a mobile node has recently heard, thereby yielding greater knowledge of network connectivity. AODV is designed for unicast routing only, and multi-path is not supported. In other words, only one route to a given destination can exist at a time. However, enhancements have been proposed which extend the base AODV to provide multi-path capability, and it is known as Multipath AODV (MAODV). 2.3.2.3 Link Reversal Routing and TORA The Temporally Ordered Routing Algorithm (TORA) [Park1997] is a highly adaptive loop-free distributed routing algorithm based on the concept of link reversal. It is designed to minimize reaction to topological changes. A key design concept in TORA is that it decouples the generation of potentially far-reaching control messages from the rate of topological changes. Such messaging is typically localized to a very small set of nodes near the change without having to resort to a complex dynamic, hierarchical routing solution. Route optimality (shortest-path) is considered of secondary importance, and longer routes are often used if discovery of newer routes could be avoided. TORA is also characterized by a multi-path routing capability. Each node has a height with respect to the destination that is computed by the routing protocol. Figure 2.5 illustrates the use of the height metric. It is simply the distance from the destination node. K. V. Pradeep

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Routing in Ad-Hoc Networks TORA is proposed to operate in a highly dynamic mobile networking environment. It is source initiated and provides multiple routes for any desired source/destination pair. To accomplish this, nodes need to maintain routing information about adjacent (one-hop) nodes. The protocol performs three basic functions: • Route creation, • Route maintenance, and • Route erasure.

From each node to each destination in the network, a separate directed acyclic graph (DAG) is maintained. When a node needs a route to a particular destination, it broadcasts a QUERY packet containing the address of the destination for which it requires a route. This packet propagates through the network until it reaches either the destination, or an intermediate node having a route to the destination. The recipient of the QUERY then broadcasts an UPDATE packet, listing its height with respect to the destination. As this packet propagates through the network, each node that receives the UPDATE sets its height to a value greater than the height of the neighbor from which the UPDATE has been received. This has the effect of creating a series of directed links from the original sender of the QUERY to the node that initially generated the UPDATE. When a node discovers that a route to a destination is no longer valid, it adjusts its height so that it is a local maximum with respect to its neighbors and transmits an UPDATE packet. If the node has no neighbors of finite height with respect to this destination, then the MH attempts to discover a new route as described above. When a node detects a network partition, it generates a CLEAR packet that resets routing state and removes invalid routes from the network.

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TORA is layered on top of IMEP, the Internet MANET Encapsulation Protocol which is required to provide reliable, in-order delivery of all routing control messages from a node to each of its neighbors, plus notification to the routing protocol whenever a link to one of its neighbors is created or broken. To minimize overhead, IMEP aggregates many TORA and IMEP control messages (which IMEP refers to as objects) together into a single packet (as an object block) before transmission. Each block carries a sequence number and a response list of other nodes from which an ACK has not yet been received, and only those nodes acknowledge the block when receiving it; IMEP retransmits each block with some period, and continues to retransmit it if needed for some maximum total period, after which TORA is notified of each broken link to unacknowledged nodes. For link status sensing and maintaining a list of a node's neighbors, each IMEP node periodically transmits a BEACON packet, which is answered by each node hearing it with a HELLO packet. During the route creation and maintenance phases, nodes use the "height" metric to establish a DAG rooted at the destination. Thereafter, links are assigned a direction (upstream or downstream) based on the relative height metric of neighboring nodes as shown in Figure 2.6(a). When node mobility causes the DAG route to be broken, route maintenance becomes necessary to reestablish a DAG rooted at the same destination. As shown in Figure 2.6(b), upon failure of the last downstream link, a node generates a new reference level that effectively coordinates a structured reaction to the failure. Links are reversed to relect the change in adapting to the new reference level. Timing is an important factor for TORA because the "height" metric is dependent on the logical time of a link failure; TORA assumes that all nodes have synchronized clocks (accomplished via an external time source such as the Global Positioning System). TORA’s metric comprises of quintuple elements, namely: • Logical time of a link failure, • The unique ID of the node that defined the new reference level, • A relection indicator bit, • A propagation ordering parameter, • The unique ID of the node.

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The first three elements collectively represent the reference level. A new reference level is defined each time a node loses its last downstream link due to a link failure. TORA’s route erasure phase essentially involves flooding a broadcast clear packet (CLR) throughout the network to erase invalid routes. In TORA, oscillations might occur, especially when multiple sets of coordinating nodes concurrently detect partitions, erase routes, and build new routes based on each other (Figure 2.7). Because TORA uses inter-nodal coordination, its instability is similar to the "count-to-infinity" problem, except that such oscillations are temporary and the route ultimately convergences. Note that TORA is partially proactive and partially reactive. It is reactive in the sense that route creation is initiated on-demand. However, route maintenance is done on a proactive basis such that multiple routing options are available in case of link failures.

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Routing in Ad-Hoc Networks 2.3.3 Hybrid Routing Approach Even though sometimes not explicit, most hybrid protocols do try to employ some sort of hierarchical arrangement (or pseudo hierarchy). Usually, this hierarchy can be based either on the neighbors of a node or in different partitions of the network. 2.3.3.1 Zone Routing Protocol Zone Routing Protocol (ZRP) is an example of hybrid reactive and proactive schemes. It limits the scope of the proactive procedure only to the node's local neighborhood, while the search being global throughout the network can be performed efficiently by querying selected nodes in the network, as opposed to querying all the network nodes. ZRP can be said to be a neighbor selection based protocol. A node employing ZRP proactively maintains routes to destinations within a local neighborhood, which is referred to as a routing zone and is defined as a collection of nodes whose minimum distance in hops from the node in question is no greater than a parameter referred to as zone radius. Each node maintains its zone radius and there is an overlap between neighboring zones. The construction of a routing zone requires a node to first know who its neighbors are. A neighbor is defined as a node that can communicate directly with the node in question and is discovered through a MAC level Neighbor discovery protocol (NDP). The ZRP maintains routing zones through a proactive component called the Intrazone routing protocol (TARP) which is implemented as a modified distance vector scheme. On the other hand, the Interzone routing protocol (IERP) is responsible for acquiring routes to destinations that are located beyond the routing zone. The IERP uses a queryresponse mechanism to discover routes ondemand. The TERP is distinguished from the standard looding algorithm by exploiting the structure of the routing zone, through a process known as bordercasting. The ZRP provides this service through a component called Border resolution protocol (BRP). Border cast is more expensive than the broadcast flooding used in other reactive protocols. Nodes generally have many more border nodes than neighbors. In addition, each bordercast message has to traverse zoneradius hops to the border. Therefore, ZRP proposes a number of mechanisms to reduce the cost of bordercast route requests [Haas1998a]. Redundancy suppressing mechanisms based on caching overhead traffic include query detection, early termination and loop back termination. The TARP topology K. V. Pradeep

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Routing in Ad-Hoc Networks information maintained at each node can be used for backward search prevention and selective bordercasting. Selective bordercasting is similar to the MPR selection used in OLSR; each node selects a subset of its border nodes that achieves equivalent coverage. The network layer triggers an IERP route query when a data packet is to be sent to a destination that does not lie within its routing zone. The source generates a route query packet, which is uniquely identified by a combination of the source node's ID and the request number. The query is then broadcast to all the peripheral nodes of the source. Upon receipt of a route query packet, a node adds its ID to the query. The sequence of recorded node IDs specifies an accumulated route from the source to the current routing zone. If the destination does not appear in the node's routing zone, the node bordercasts the query to its peripheral nodes. If the destination is a member of the routing zone, a route reply is sent back to the source, along the path specified by reversing the accumulated route. A node discards any route query packet for a query that it has previously encountered. An important feature of this route discovery process is that a single route query can return multiple route replies. The quality of these returned routes can be determined based on some metric. Then, the relative quality of the route can be used to select the best route. Route failure is detected proactively, in conjunction with the IARP. Failures may be repaired locally, in which case it may not even be necessary to inform the source node. If necessary, a hop-limited local request can be used to repair the route, or a route error message can be set to re-initiate the route discovery from the source. An adaptive and distributed configuration of each node's routing zone in ZRP provides a lexible solution [Samar2004]. This is possible by incorporating local characteristics such as local route information for global route discovery, etc. A substantial improvement is observed that enhances the network scalability and routing robustness. 2.3.3.2 Fisheye State Routing The Fisheye State Routing (FSR) protocol introduces the notion of multi-level fisheye scope to reduce routing update overhead in large networks. Nodes exchange link state entries with their neighbors with a frequency that depends on distance to destination. From link state entries, nodes construct the topology map of the entire network and compute optimal routes. FSR tries to improve the scalability of a routing protocol by putting most efforts in gathering data on the topology information that is most likely to be needed soon. Assuming that nearby changes to the network topology are those most K. V. Pradeep

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Routing in Ad-Hoc Networks likely to matter, FSR tries to focus its view on nearby changes by observing them with the highest resolution in time and changes at distant nodes are observed with a lower resolution and less frequently. It is possible to interpret the FSR as the one blurring the sharp boundary defined by the ZRP model. 2.3.3.3 Landmark Routing (LANMAR) for MANET with Group Mobility Landmark Ad Hoc Routing (LANMAR) combines the features of FSR and Landmark routing. The key feature is the use of landmarks for each set of nodes which move as a group (e.g., a group of soldiers in a battlefield) in order to reduce routing update overhead. Like FSR, nodes exchange link state only with their neighbors. Routes within Fisheye scope are accurate, while routes to remote groups of nodes are "summarized" by the corresponding landmarks. A packet directed to a remote destination, initially aims at the landmark; as it gets closer to destination it eventually switches to the accurate route provided by Fisheye. In the original wired landmark scheme , predefined hierarchical address of each node relects its position within the hierarchy and helps find a route to it. Each node knows the routes to all the nodes within its hierarchical partition. Moreover, each node knows the routes to various "landmarks" at different hierarchical levels. Packet forwarding is consistent with the landmark hierarchy and the path is gradually refined from top-level hierarchy to lower levels as a packet approaches the destination. LANMAR borrows the notion of landmarks [Tsuchiyal988] to keep track of logical subnets. A subnet consists of members which have a commonality of interests and are likely to move as a "group" (e.g., soldiers in the battlefield). A "landmark" node is elected in each subnet. The routing scheme itself is a modified version of FSR. The main difference is that the FSR routing table contains "all" nodes in the network, while the LANMAR routing table includes only the nodes within the scope and the landmark nodes. This feature greatly improves scalability by reducing routing table size and update traffic overhead. When a node needs to relay a packet, if the destination is within its neighboring scope, the address is found in the routing table and the packet is forwarded directly. Otherwise, the logical subnet field of the destination is searched and the packet is routed towards the landmark for that logical subnet. The packet, however, does not need to pass through the landmark. Rather, once the packet gets within the scope of the destination, it is routed directly. The routing update exchange in LANMAR routing is similar to FSR. Each node periodically exchanges topology information with its immediate neighbors. In each update, the node sends entries within its fisheye scope. It K. V. Pradeep

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Routing in Ad-Hoc Networks also piggybacks a distance vector with size equal to the number of logical subnets and thus landmark nodes. Through this exchange process, the table entries with larger sequence numbers replace the ones with smaller sequence numbers. 2.3.3.4 Cluster-Based Routing Protocol The Cluster-Based Routing Protocol (CBRP) is a partitioning protocol emphasizing support for unidirectional links Clusters are defined by bidirectional links, but inter-cluster connectivity may be obtained via a pair of unidirectional links. Each node maintains two-hop topology information to define clusters. Each cluster includes an elected cluster head, with which each member node has a bi-directional link. Clusters may be overlapping or disjoint; however, cluster-heads may not be adjacent. In addition to exchanging neighbor information for cluster formation, nodes must find and inform their cluster head(s) of the status of the "gateway" nodes, cluster members which can be reached from a node belonging to another cluster. Thus, each cluster-head has knowledge of all the clusters with which it has bi-directional connectivity, possibly via a pair of unrelated unidirectional links. The latter are discovered by flooding adjacent cluster heads with a request for an appropriate link. When a source has no route to a destination, it forwards a route request to its cluster head. The cluster infrastructure is used to reduce the cost of disseminating the request. When a cluster-head receives a request, it appends to the request packet its ID, as well as a list of (non-redundant) adjacent clusters, and rebroadcasts it. Each neighboring node which is a gateway to one of these adjacent clusters unicasts the request to the appropriate cluster head. When the request reaches the destination, it contains a loose source routing specifying a sequence of clusters. When the route reply is sent from the destination back to the source, each intermediate cluster head writes a complete source route into the reply, optimizing that portion of the route based on its knowledge of cluster topology. Therefore, routes need not pass through cluster heads. When the complete source route is received at the source, it is used for data traffic. As with DSR, intermediate nodes may generate new routes to take advantage of improved routes or salvaged failed routes. Unlike DSR, only cluster-level (two-hop neighborhood) information may be used for this purpose: nodes do not attempt to cache network-scale topology information.

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Routing in Ad-Hoc Networks 2.3.4 Comparison Table 2.1 summarizes the main characteristics of some of the most prominent topology-based protocols discussed so far. The criteria used for comparison are self-explanatory and have been extensively covered in the previous sections. Routing Route Protocol Acquisition

DSDV Computed a

Flood for Route Discovery

Delay for Route Discovery

Multi-Path Capability

No

No

No

Floods route updates throughout the network

No

No

No

Ultimately, updates the routing table of all nodes by exchanging MRL B/w neighbors

Yes. Aggressive use of caching attempts to reduce route discovery delay

Yes

Not Explicitly. The technique of salvaging may quickly restore a route

Route error propagated up to the source to erase invalid path

Yes,

Yes

Not Directly, However, the Multipath AODV Protocol includes this support

Route error broadcasted to erase invalid path

Yes

Error is recovered locally, and only when alternative routes are not available

No

Hybrid of Updating nodes tables within a zone and propagating route error to the source.

Priori

WRP Computed a Priori

DSR On-Demand only when needed

AODV On-Demand, only when needed

Conservative use of cache to reduce route discovery delay

TORA On-Demand, Usually, only Yes, once the

ZRP

only when needed

one flood for initial DAG Construction

DAG is Constructed, Multiple paths are found

Hybrid

Only outside a sources zone

Onlye if the destination is outside the sources zone

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2.4 Position-Based Routing In this section we discuss some ad hoc routing protocols that take advantage of some sort of location information in the routing process [Mauve2001]. Before delving into the forwarding schemes, it is of paramount importance to discuss the principles and issues behind position-based routing, as well as to look into location services. 2.4.1 Principles and Issues The philosophy of position-based routing is that it is necessary to determine the location of the destination before a packet can be sent. Generally, a location service takes this responsible. Existing location services can be classified according to how many MHs have the service. This can be either some specific nodes or all the network nodes. Moreover, each location server may maintain the position of some specific nodes or all the nodes in the network. In the following discussion on location services, we consider all four possible combinations of some-for-some, some-for-all, all-for-some, and all-for-all MHs. In position-based routing, the forwarding decision by a MH is essentially based on the position of a packet's destination and the position of the node's immediate one-hop neighbor. Clearly, the position of the destination is contained in the header of the packet. If a node happens to know an accurate position of the destination, it may choose to update the position of the packet before forwarding it. The position of the neighbors is typically learned through one-hop broadcasts. These beacons are sent periodically by all nodes and contain the position of the sending node. Three main packet forwarding schemes can be defined for positionbased routing: • Greedy forwarding; • Restricted directional flooding; • Hierarchical approaches. For the first two, a node forwards a given packet to one (greedy forwarding) or more (restricted directional looding) one-hop neighbors that are located closer to the destination than the forwarding node itself. The selection of the neighbor in the greedy case depends on the optimization criteria of the algorithm. It is fairly obvious that both forwarding strategies may fail if there is no one-hop neighbor that is closer to the destination than the forwarding node itself. Recovery strategies that cope with this kind of failure are also discussed later in this chapter. K. V. Pradeep

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Routing in Ad-Hoc Networks The third forwarding strategy is to form a hierarchy in order to scale to a large number of MHs. In this chapter we investigate two representatives of hierarchical routing that use greedy forwarding for wide area routing and non-position based approaches for local area routing. Figure 2.8 depicts the two main building blocks, namely, location service and forwarding strategy, that are required for position-based routing. In addition, we illustrate potential classification criteria for the various existing approaches.

2.4.2 Location Services In order to learn the current position of a specific node, help is needed from a location service. MHs register their current position with this service. When a node does not know the position of a desired communication partner, it contacts the location service and requests that information. In classical one-hop cellular networks, there are dedicated position servers (with well-known addresses) that maintain position information about the nodes in the network. With respect to classification, this is some-forall approach as the servers are some specific nodes, each maintaining the position information about all MHs. In MANETs, such centralized approach is viable only as an eternal service that can be reached via non-ad hoc means. There are two main reasons for this. First, it would be difficult to obtain the location of a position server if the server is a part of the MANET itself. This would represent a chicken-andegg problem: without the position server it is not possible to get position information, but without the position information the server cannot be reached. Second, since a MANET is dynamic, it might be difficult to guarantee that at least one position server will be present in a given MANET. In the following, we concentrate on decentralized location services that are part of the MANET.

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Routing in Ad-Hoc Networks 2.4.2.1 Distance Routing Effect Algorithm for Mobility Within Distance Routing Effect Algorithm for Mobility (DREAM) framework, each node maintains a position database that stores the location information about other nodes that are part of the network. As a consequence, it can be classified as an all-for-all approach. An entry in the position database includes a node identifier, the direction of and distance to the node, as well as a time value that indicates when this information has been generated. Obviously, the accuracy of such an entry depends upon its age. Each node running DREAM periodically floods packets to update the position information maintained by the other nodes. A node can control the accuracy of its position information available to other nodes in two ways: • By changing the frequency at which it sends position updates. This is known as temporal resolution; •

By indicating how far a position update may travel before it is discarded. This is known as spatial resolution.

The temporal resolution of sending updates is coupled with the mobility rate of a node, i.e., the higher the speed is, more frequent the updates will be. The spatial resolution is used to provide accurate position information in the direct neighborhood of a node and less accurate information at nodes farther away. The costs associated with accurate position information at remote nodes can be reduced since greater the distance separating two nodes is, slower they appear to be moving with respect to each other. Accordingly, the location information in routing tables can be updated as a function of the distance separating nodes without compromising the routing accuracy. This is called as the distance effect and is exemplified by Figure 2.9 where MH A is assumed stationary, while MHs B and C are moving in the same direction at the same speed. From node A's perspective, the change in direction will be greater for node B than for node C. The distance effect allows low spatial resolution areas far away from the target node, provided that intermediate hops are able to update the position information contained in the packet header. Based on the resulting routing tables, DREAM forwards packets in the recorded direction of the destination node, guaranteeing delivery by following the direction with a given probability.

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2.4.2.2 Quorum-Based Location Service The concept of quorum systems is quite popular in distributed systems and information replication in databases. Here, information updates (write operations) are sent to a subset (quorum) of available nodes, and information requests (read operations) are referred to a potentially different subset. When these subsets are designed such that their intersection is nonempty, it is ensured that an up-to-date version of the sought-after information can always be found. It is employed to develop a location service for MANETs. It is instructive to discuss this scheme through a sample network shown in Figure 2.10. A set of MHs is chosen to host position databases, and this is illustrated by nodes 1-6 in Figure 2.10. Next, a virtual backbone is constructed among the nodes of the subset by utilizing a non-position-based ad hoc routing algorithm. A MH sends position update messages to the nearest backbone node, which then chooses a quorum of backbone nodes to host the position information. In our example, node D sends its updates to node 6, which might then select quorum A with nodes 1, 2, and 6 to host the information. When a node S wants to obtain the position information, it sends a query to the nearest backbone node, which in turn contacts (through unicast or even multicast) the nodes of a (usually different) quorum. Node 4 might, for example, choose quorum B, consisting of nodes 4, 5, and 6 for the query. Since, by definition, the intersection of two quorum systems is nonempty, the querying node is guaranteed to obtain at least one response with the desired position information. It is important to timestamp position updates. If several responses are received, the one representing the most current position update is selected.

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An important trade-off in quorum-based position services is that larger the quorum set is, higher the cost for position updates and queries are, while larger the number of nodes in the intersection of two quorums will be. This improves resilience against unreachable backbone nodes. Several methods on how to generate quorum systems with desired properties can be found in [Haas1999]. The quorum-based position service can be configured to operate as all-for-all, all-for-some, or somefor-some approach, depending upon how the size of the backbone and the quorum is chosen. However, it will typically work as some-for-some scheme with the backbone being a small subset of all available nodes and a quorum being a small subset of the backbone nodes. Another work based on quorums in presented in Here, position information for the nodes is propagated in a north-south direction. Whenever a node has to be contacted whose position is not known, position information is searched in east-west direction until the information is found. 2.4.2.3 Grid Location Service The Grid Location Service (GLS) divides the area that contains the MANET into a hierarchy of squares. In this hierarchy, n-order squares contain exactly (n — 1)-order squares, forming a so-called quadtree. Each node maintains a table of all other nodes within the local first-order square. The table is constructed with the help of periodic position broadcasts scoped to the area of the first order square. We present GLS with the assistance of Figure 2.11. To determine where to store position information, GLS establishes a notion of near node IDs, defined as the least ID greater than a node's own ID. When node 10 in Figure 2.11 wants to distribute its position information, it sends position updates to the respective node with the nearest ID in each of the three K. V. Pradeep

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Routing in Ad-Hoc Networks surrounding first-order squares. Therefore, the position information is available at nodes 15, 18, 73, and at all nodes that are in the same first-order square as node 10 itself. In the surrounding three second-order squares, again the nodes with the nearest ID are selected to host the node's position (nodes 14, 25, and 29 in the example of Figure 2.11). This process is repeated until the area of the MANET has been fully covered. As we can see, the density of the position information for a given node decreases logarithmically with the distance from that node. Now assume that node 78 wants to obtain the position of node 10. Firstly, it should locate a nearby node that knows about the position of node 10. In our example, this is node 29. While node 78 does not know that node 29 possesses the required position, it is able to discover this information. To understand how this process works, it is important to look at the position servers of node 29. Its position is stored in the three surrounding first-order squares at nodes 36, 43, and 64. Note that each of these nodes, including node 29

are also automatically the ones in their respective first-order square with the ID nearest to 10. Thus, there exists a "trail" of descending node IDs from each of the squares of all orders to the correct position server. Position queries for a node can now be directed to the node with the nearest ID of which the querying node knows. In our example, this would be node 36. The node with the nearest ID does not necessarily know the node sought, but will know the node with a nearer node ID. This would be node 29 in our example, which happens to be the sought position server. This process continues until a node that has the position information available is found. Note that a node need not know the IDs of its position servers. Position information is forwarded to a certain position (e.g., the lower left corner) of each element in the quadtree and is then forwarded progressively to nodes with closer IDs to ensure that the position information reaches the correct node. Since GLS requires that all nodes store the information on some other nodes, it can therefore be classified as an all-for-some approach. K. V. Pradeep

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Routing in Ad-Hoc Networks 2.4.2.4 Homezone Two almost identical location services have been proposed independently in [Giordano1999, Stojmenovic1999b]. Both use the concept of a virtual Homezone where position information for a node is stored. By applying a well-known hash function to the node identifier, it is possible to derive the position C of the Homezone for a node. All nodes within a disk of radius R centered at C have to maintain position information for the node. Thus, as in the case of GLS, a position database can be found by means of a hash function on which sender and receiver agree without having to exchange information. If the Homezone is sparsely populated, R may have to be increased, resulting in increasing R for updates as well as for queries. Therefore the Homezone approaches are also all-for-some approaches. 2.4.3 Forwarding Strategies In this section we describe the three major forwarding strategies employed in position-based routing. 2.4.3.1 Greedy Packet Forwarding Using greedy packet forwarding, the sender of a packet includes an approximate position of the recipient in the packet. This information is gathered by an appropriate location service (e.g., described in the previous section). When an intermediate node receives a packet, it forwards the packet to a neighbor lying in the general direction of the recipient. Ideally, this process can be repeated until recipient has been reached. Typically, there are three different strategies a node can use to decide to which neighbor a given packet should be forwarded. These are illustrated in Figure 2.12, where node S and D denote the source and destination nodes of a packet, respectively.

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Routing in Ad-Hoc Networks The circle with radius r indicates the maximum transmission range of node S. One intuitive strategy is to forward the packet to the node that makes the most progress towards (i.e., closest to) node D. In Figure 2.12, this would be node C. This strategy is known as most forward within r (MFR) tries to minimize the number of hops a packet has to transverse in order to reach node D. MFR may be a good strategy in scenarios where the sender of a packet cannot adjust the transmission signal strength to the distance between the sender and receiver. However, it is shown that a different strategy performs better than MFR in situations where the sender can adapt its transmitting power. In nearest with forward progress (NFP), the packet is transmitted to the nearest neighbor of the sender which is in the direction of the destination. In Figure 2.12, this would be node A. If all nodes employ NFP, the probability of packet collisions is significantly reduced. Thus, the average progress of the packet is calculated as p f(a, b) where p is the likelihood of a successful transmission without collision and f(a, b) is the progress of the packet being successfully forwarded from a to b and is higher for NFP than for MFR. Another strategy for forwarding packets is compass routing, in which the neighbor closer to the straight line between sender and destination is selected In our example of Figure 2.12, this would be node B. Compass routing tries to minimize the spatial distance a packet travels. Finally, it is possible to let the sender randomly select one of the nodes closer to the destination than itself and forward the packet to that node [Nelson1984]. This strategy minimizes the accuracy of information needed about the position of the neighbors and reduces the number of operations required to forward a packet. Unfortunately, greedy routing may fail to find a path between a sender and a destination, even though one does exist. This can be seen through Figure 2.13, where the circle around node D has the radius of the distance between nodes S and D, and circle around node S shows its transmission range. Note that there exists a valid path from node S to node D. The problem here is that node S is closer to the destination node D than any of the nodes in its transmission range. Greedy routing has therefore reached a local maximum from which it cannot recover.

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To counter this problem, it has been suggested that the packet should be forwarded to the node with the least backward (negative) progress if no node can be found in the forward direction. However, this raises the problem of looping, which cannot occur when packets are forwarded only toward the destination with positive progress. Other studies suggest not to forward packets that have reached a local maximum. The face-2 algorithm and the perimeter routing strategy of the Greedy Perimeter Stateless Routing Protocol (GPSR) are two similar recovery approaches based on planar graph traversal. Both are performed on a perpacket basis and do not require nodes to store any additional information. A packet enters the recovery mode when it arrives at a local maximum. It returns to greedy mode when it reaches a node closer to the destination than the node where the packet entered the recovery mode. Planar graphs are graphs with non-intersecting edges. A set of nodes in a MANET can be considered a graph in which the nodes are vertices and an edge exists between two vertices if they are close enough to communicate directly with each other. The graph formed by a MANET is generally not planar, and an example is in Figure 2.14 where the transmission range of each node contains all other nodes.

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Routing in Ad-Hoc Networks In order to construct a connected planar subgraph of the graph formed by the nodes in a MANET, a well-known mechanism is employed an edge between two nodes A and B is included in the graph only if the intersection of the two circles with radii equal to the distance between node A and B around those two nodes does not contain any other nodes. For example, in Figure 2.14 the edge between nodes A and C would not be included in the planar subgraph since nodes B and D are contained in the intersection of the circles. It is important to realize that each node can locally make the decision as to whether an edge is within the planar subgraph, since each node knows the position of all its neighbors. Based on the planar subgraph, a simple planar graph traversal is used to find a path toward the destination. The general concept is to forward the packet on faces of the planar subgraph progressively closer to the destination. Figure 2.15 shows how this traversal is carried out when a packet is forwarded from node S toward node D on recovery mode.

On each face, the packet is forwarded along the interior of the face by using the right hand rule: forward the packet on the next edge counterclockwise from the edge on which it arrived. Whenever the line between source and destination intersects the edge along which a packet is about to be forwarded, check if this intersection is closer to the destination than any other intersection previously encountered. If this is true, switch to the new face bordering the edge the packet is about to transverse. The packet is then forwarded on the next edge counterclockwise to the edge it is about to be forwarded along before switching faces. This algorithm guarantees that a path will be found from the source to the destination in case at least one exists in the original non-planar graph.

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Routing in Ad-Hoc Networks The header of a packet contains additional information such as the position of the node where it entered recovery mode, the position of the last intersection that caused a face change, and the first edge traversed on the current face. Therefore, each node can make all routing decisions based only on the information about its local neighbors. This includes detection of an unreachable destination, when a packet traverses an earlier visited edge for the second time. 2.4.3.2 Restricted Directional Flooding 2.4.3.2.1 DREAM In DREAM the sender node S of a packet with destination node D forwards the packet to all one-hop neighbors that lie "in the direction of node D". In order to determine this direction, a node calculates the region that is likely to contain node D, called the expected region. As depicted in Figure 2.16, the expected region is a circle around the position of node D as it is known by node S. Since this position information may be outdated, the radius r of the expected region is set to (t1 — to)vni , where t1 is the current time, to is the timestamp of the position information node S has about node D, and v,,„,„ is the maximum speed that a node may travel in the MANET. Given the expected region, the "direction towards node D" for the example in Figure 2.16 is defined by the line between nodes S and D and the angle 0. The neighboring nodes repeat this procedure using their information on node D's position. If a node does not have a one-hop neighbor in the required direction, a recovery procedure has to be started. This procedure is not part of DREAM specification. 2.4.3.2.2 Location-Aided Routing The Location-Aided Routing (LAR) protocol does not define a locationbased routing protocol, but instead proposes the use of position information to enhance the route discovery phase of reactive ad hoc routing approaches, which often use flooding as a means of route discovery. Under the assumption that nodes have information about other node's positions, LAR uses this position information to restrict the flooding to a certain area. This is carried out similar to DREAM.

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Routing in Ad-Hoc Networks LAR exploits location information to limit the scope of route request flood employed in protocols such as AODV and DSR. Such location information can be obtained, for example, through GPS. LAR limits the search for a route to the so-called request zone, determined based on the expected location of the destination node at the time of route discovery. Two concepts are important to understand how LAR works: Expected Zone and Request Zone. Let us first discuss what an Expected Zone is. Consider a node S that needs to find a route to node D. Assume that node S knows that node D was at location L at time to. Then, the "expected zone" of node D, from the viewpoint of node S at current time ti, is the region expected to contain node D. For instance, if node S knows that node D travels with average speed v, then S may assume that the expected zone is the circular region of radius v(ti - to), centered at location L (see Figure 2.17(a)). If actual speed happens to be larger than the average, then the destination may actually be outside the expected zone at time t1. Thus, expected zone is only an estimate made by node S to determine a region that potentially contains D at time t1. If node S does not know any previous location of node D, then node S cannot reasonably determine the expected zone (the entire region that may potentially be occupied by the ad hoc network is assumed to be the expected zone). In this case, LAR reduces to the basic looding algorithm. In general, having more information regarding mobility of a destination node can result in a smaller expected zone as illustrated by Figure 2.17(b).

Based on the expected zone, we can define the request zone. The proposed LAR algorithms use flooding with one modification. Node S defines (implicitly or explicitly) a request zone for the route request. A node forwards a route request only if it belongs to the request zone (unlike the flooding algorithm in AODV and DSR). To increase the probability that the route request will reach node D, the request zone should include the expected zone (described above). Additionally, the request zone may also include other regions around the request zone. K. V. Pradeep

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Based on this information, the source node S can thus determine the four corners of the expected zone. For instance, in Figure 2.18 if node I receives the route request from another node, node I forwards the request to its neighbors, because I determines that it is within the rectangular request zone. However, when node J receives the route request, node J discards the request, as node J is not within the request zone (see Figure 2.18).

This algorithm is called LAR scheme 1. The LAR scheme 2 is a slight modification to include two pieces of information within the route request packet: assume that node S knows the location (Xd; Yd) of node D at some time to — the time at which route discovery is initiated by node S is t1, where t1 to. Node S calculates its distance from location (Xd; Yd), denoted as DISTs, and includes this distance with the route request message. The coordinates (Xd; Yd) are also included in the route request packet. With this information, a given node J forwards a route request forwarded by I (originated by node S), if J is within an expected distance from (Xd;Yd) than node I. 2.4.3.2.3 Relative Distance Micro-Discovery Ad Hoc Routing The Relative Distance Micro-discovery Ad Hoc Routing (RDMAR) routing protocol, an adaptive and scaleable routing protocol, is well suited in large mobile networks whose rate of topological changes is moderate. A key concept in its design is a typical localized reaction to link failures to a very small region of the network near the change. This desirable behavior is achieved through the use of a flooding mechanism for route discovery, called Relative Distance Microdiscovery (RDM). To accomplish this, an iterative algorithm K. V. Pradeep

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Routing in Ad-Hoc Networks calculates an estimate of their RD given their previous RD, an average nodal mobility and information about the elapsed time since they last communicated. Based on the newly calculated RD, the query lood is then localized to a limited region of the network centered at the source node of the route discovery and with maximum propagation radius that equals to the estimated relative distance. In RDMAR, packets are routed between the stations of the network by using routing tables which are stored at each station of the network. Each routing table lists all reachable destinations, wherein for each destination j, it includes: the "Default Router" field that indicates the next hop node through which the current node can reach j, the "RD" field which shows an estimate of the relative distance (in hops) between the node and j, the "Time_Last_Update" (TLU) field that indicates the time since the node last received routing information for j, a "RT Timeout" field which records the remaining amount of time before the route is considered invalid, and a "Route Flag" field which declares whether the route to j is active. RDMAR comprises of two main algorithms: •

Route Discovery — When an incoming call arrives at node i for destination node j and there is no route available, i initiates a route discovery phase. Here, i has two options; either to flood the network with a route query in which case the route query packets are broadcast into the whole N/W or instead, limit the discovery in a smaller region of the network, if some kind of location prediction model for j can be established. In the latter case, the source of the route discovery, i, refers to its routing table in order to retrieve information on its previous relative distance with j and the time elapsed since i last received routing information for j. Let us designate this time as tmotion. Based on this information and assuming a moderate velocity, Micro_Velocity, and a moderate transmission range, Micro_Range, node i is then able to estimate its new relative distance to destination node j in terms of actual number of hops. To accomplish this, node i calculates the distance offset of DST (DST Offset) during tnx„,0„, and "adjusts" the result onto their previous relative distance (RDM_Radius). • Route Maintenance — An intermediate node i, upon receipt of a data packet, first processes the routing header and then forwards the packet to the next hop. In addition, node i sends an explicit message to examine whether a bidirectional link can be established with the previous node. RDMAR, therefore, does not assume bi-directional links but in contrast nodes exercise the K. V. Pradeep

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Routing in Ad-Hoc Networks possibility of having bidirectional links. If node i is unable to forward the packet because there is no route available or a forwarding error occurs along the data path as a result of a link or node failure, i may attempt a number of additional re-transmissions of the same data packet, up to a maximum number of retries. However, if the failure persists, node i initiates a Route Discovery procedure. 2.4.3.3 Hierarchical Routing In traditional networks, the complexity of the routing algorithm handled by each node can be reduced tremendously by establishing some form of hierarchy. Therefore, it is a valid question to ask whether position-based routing for MANETs can also benefit from the use of hierarchy. 2.4.3.3.1 Terminodes Routing One approach that combines hierarchical and position-based routing is a part of the Terminodes project with two levels of hierarchy. Packets are routed according to a proactive distance vector scheme if the destination is close (in terms of number of hops) to the sending node. For long distance routing, a greedy positionbased approach is used. Once a long distance packet reaches the area close to the recipient, it continues to be forwarded by means of the local routing algorithm. It is shown by simulations in [Blazevic2000] that the hierarchy can significantly improve the ratio of successfully delivered packets and the routing overhead compared to conventional reactive ad hoc routing protocols. In order to prevent greedy forwarding for long distance routing from encountering a local maximum, the sender includes a list of positions in the packet header which are then traversed on its way to the sender. In Terminodes routing, the sender requests this information from nodes it is already in contact with (e.g., the nodes that are reachable using the local routing protocol). Once a sender has this information, it needs to check at regular intervals whether the path of positions is still valid or can be improved. 2.4.3.3.2 Grid Routing A second method for position-based ad hoc routing containing hierarchical elements is proposed within the Grid project. The location proxy technique described in is similar to the Terminodes routing: a proactive distance vector routing protocol is used at the local level, while positionbased routing is employed for long-distance packet forwarding. In Grid K. V. Pradeep

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Routing in Ad-Hoc Networks routing, however, the hierarchy is not only introduced to improve scalability. The main idea here is to have at least one position-aware node in each area to be used as proxies. Packets that are addressed to a position-unaware node therefore arrive at a position-aware proxy and are then forwarded according to the information of the proactive distance vector protocol. As a repair mechanism for greedy long-distance routing, a mechanism called Intermediate Node Forwarding (INF) is proposed. If a forwarding node has no neighbor with forward progress, it discards the packet and sends a notification to the sender of the packet. The sender of the packet then chooses a single intermediate position randomly for a circle around the midpoint of the line between the sender and the receiver. Packets have to traverse that intermediate position. If the packet is discarded again, the radius of the circle is increased and another random position is chosen. This is repeated until the packets are delivered to the destination, or until a predefined number has been attempted when the sender assumes that the destination is unreachable. 2.4.3.4 Other Position-Based Routing Effectiveness of all position-based routing depends on the accuracy of the location of the destination node. The GPS-based systems do not provide good accuracy inside the building and the surrounding area can be classified in the following five categories: • Typical office environment with no line-of-sight (NLOS) with 5Ons delay spread. • Large open space with 100ns delay spread with NLOS. • Large indoor or outdoor space with 150ns delay spread with NLOS. • Large indoor or outdoor space with line-of-sight & 140ns delay spread. • Large indoor or outdoor space with NLOS and 250ns delay spread. The instantaneous received signal strength for a fixed location inside a building is observed to vary with time due to shadow, fading and multi-path reception. The closest neighbors' location is observed to provide good accuracy and reasonable performance under all categories. Existing location based routing schemes use the last known destination location to the source as the best zone estimate. Therefore, it is better to combine location-based routing with specific geographical points known as anchors as selected by the source. These imaginary locations assist in routing and are selected based on those nodes that could possibly assist in path discovery or could be based on geographical node density maps at the source node. K. V. Pradeep

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2.4.4 Comparisons We compare the location services and forwarding strategies previously described. One key aspect of this comparison is how the individual approaches behave with an increasing number of nodes in the MANET. Next, we assume that the density of nodes remain constant when the number of nodes increases. Therefore, the area covered by the MANET has to increase as the number of nodes increases. 2.4.4.1 Location Services A comparison between different location-based routing is given in Table 2.2 summarizes various location services using several different criteria, where n represents the number of nodes and c is a constant. The type criterion indicates how many nodes participate in providing location information and for how many other nodes each node is required to maintain location information. The communication complexity describes the average number of one-hop transmissions required to look up or update a node's position. The time complexity measures the average time it takes to perform a position update or position lookup. The amount of state required at each node to maintain the position of other nodes is indicated by the state volume. Some location services provide localized information by maintaining a higher density or better quality of position information near the position of the node. This may be important if the communication in a MANET is mainly local. The robustness of a location service is considered to be low, medium, or high, depending on whether it takes the failure of a single node, the failure of a small subset of all nodes, or the failure of all nodes to render the position of a given node inaccessible. The implementation complexity indicates how well the location service is understood and how complex it is to implement and test it. We note that this measure is highly subjective, while we have tried to be as fair as possible

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DREAM is fundamentally different from other position services, as it requires all nodes to maintain position information about every other node. The communication complexity of a position update and the position information maintained by each node scales with 0(n), while a position query requires only a local lookup, which is independent of the number of nodes. The time required to perform a position update in DREAM is a linear function of the diameter of the network, leading to a complexity of 0( In ). Due to the communication complexity of position updates, DREAM is the least scalable position service and, hence, is inappropriate for large-scale and general purpose MANETs. However, it is suitable for specialized applications since it is very robust and provides localized information in situations such as notifying an emergency. The quorum system requires the same operations for position updates and position lookups. In both cases, a constant number of nodes (the quorum) must be contacted. Each of these messages has a communication and time complexity that depends linearly on the diameter of the network and thus scales with 0(ji). The state information maintained in the backbone nodes is constant, since an individual backbone is formed for a fixed number of nodes. The general robustness of the approach is medium, since the position of a node will become unavailable if a significant number of backbone nodes fail. However, the number of such nodes is a parameter that can be freely configured for the position service. Furthermore, the position information is kept spatially distributed and independent. Thus, the robustness seems to be higher than that of GLS or K. V. Pradeep

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Routing in Ad-Hoc Networks Homezone. A major drawback of the quorum system is its dependence on a non-position-based ad hoc routing protocol for the virtual backbone, which tremendously increases the implementation complexity and may compromise the scalability of this approach. However, both position services offered by GLS and Homezone can be thought of as special case of the quorum systems, thereby overcoming this drawback. GLS and Homezone are similar to each other in that each node selects a subset of all available nodes as position servers. For Homezone, position updates and lookups need to be sent to the virtual home region (VHR). The average distance from that region depends linearly on the diameter of the network. Therefore, the communication and time complexity of Homezone is 0( V / ): The state information is constant, as each node should have a constant number of position servers in its Homezone. The performance of GLS is dependent on how the communicating nodes are distributed across the MANET. If they are uniformly distributed, the number of position servers increases logarithmically with the number of nodes. Due to the localized strategy of forwarding updates and lookups, communication and time complexity is just a constant factor larger than the Homezone and remains at 0(1/ 7 ). The main tradeoff between GLS and Homezone is in providing localized information and in the implementation complexity. GLS benefits greatly if the communicating nodes are close to each other and therefore outperforms Homezone for local communication. But, the behavior of GLS in a dynamic environment and in the presence of node failures is more difficult to control than that of Homezone. Despite of all this, we believe that both GLS and Homezone appear very promising for positioning services in general purpose MANETs. 2.4.4.2 Forwarding Strategies Table 2.3 presents a summary of various forwarding strategies and their evaluation criteria, where n represents the number of nodes. Type describes the fundamental strategy used for packet forwarding, while the communication complexity indicates the average number of one-hop transmissions required to send a packet from one node to another node with known position. The strategies need to tolerate different degrees of inaccuracy with regard to the position of the receiver and is reelected by the tolerable position inaccuracy criterion. Furthermore, the forwarding requires all-for-all location service criterion indicates whether the forwarding strategy requires all-for-all location service in order to work properly. The robustness of an approach is K. V. Pradeep

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Routing in Ad-Hoc Networks high if the failure of a single intermediate node does not prevent the packet from reaching its destination. Its value is medium if the failure of a single intermediate node might lead to the loss of the packet but does not require the setup of a new route. Finally, the robustness is low if the failure of an individual node might result in packet loss and requires setting up a new route. By definition, the position-based strategies do not maintain routes and therefore have, at least, medium robustness. As for the location service, the implementation complexity describes how complex it is to implement and test a given forwarding strategy. Greedy forwarding is efficient, with a communication complexity of 0( V ), and is well suited for use in MANETs with a highly dynamic topology. The face-2 algorithm and the perimeter routing of GPSR are currently the most advanced recovery strategies. One drawback of the current greedy approaches is that the position of the destination needs to be known with an accuracy of a one-hop transmission range, or else the packets cannot be delivered. The robustness is medium, as the failure of an individual node may cause the loss of a packet in transit. However, it does not require setting up a new route as would be the case in topology-based routing protocols. Due to repair strategy like face-2 or perimeter routing, we consider the implementation efforts to be of medium complexity.

Restricted directional flooding, as in DREAM and LAR, has communication complexity of 0(n) and therefore does not scale well for large networks with a high volume of data transmissions. One difference between DREAM and LAR is that in DREAM, it is expected that intermediate nodes update the position of the destination when they have better information than the sender of the packet, while this is not the case in LAR. As a result, DREAM packet forwarding requires and makes optimal use of all-for-all K. V. Pradeep

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Routing in Ad-Hoc Networks location service, while LAR can work with any location service but does not benefit much form an all-for-all location service if one is used. Both approaches are very robust against the failure of individual nodes and position inaccuracy, and are very simple to implement. This qualifies them for applications that require high reiability and fast message delivery for very infrequent data transmissions. Both Terminodes and Grid routing provide hierarchical approaches to position-based ad hoc routing. For long-distance routing, both use a greedy approach and therefore have characteristics similar to those of greedy forwarding. However, the use of non-position-based approach at the local level, make them tolerant to position inaccuracy, while being significantly more complex to implement. Grid routing allows position-unaware nodes to use position-aware nodes as proxies in order to participate in the MANET, while for Terminodes, a GPS-free positioning service has been developed. The probabilistic repair strategy proposed by Grid is simpler and requires less state information than that of Terminodes. Even it may fail in cases where the Terminodes succeeds in finding a path from the sender to the destination.

2.5 Other Routing Protocols There are plenty of routing protocols for MANETs, and the most important ones have been covered in detail. However, below we describe some other routing protocols which employ optimization criteria different from the ones described earlier. 2.5.1 Signal Stability Routing Unlike the algorithms described so far, the on-demand Signal Stability-Based Adaptive Routing protocol (SSR) selects routes based on the signal strength (weak or strong) between nodes and a node's location stability. This route selection criterion of SSR has the effect of choosing routes that have "stronger" connectivity. Basically, SSR is comprised of two cooperative protocols, namely, the Dynamic Routing Protocol (DRP) and the Static Routing Protocol (SRP). The DRP is responsible for the maintenance of Signal Stability Table (SST) and the Routing Table (RT). After processing the packet and updating the appropriate tables, DRP passes the packet to the SRP. The SRP of a node processes by passing the packet up the stack if it is the intended receiver, or looks up in the routing table for the destination and forwards the packet if it is not. If no entry is found in the routing table for the destination, K. V. Pradeep

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Routing in Ad-Hoc Networks a route search process is initiated. One difference between route-discoveryprocedure used in SSR with respect to that employed in AODV is that route requests are only forwarded to the next hop in SSR if they are received over strong channels. If there is no route reply received at the source within a specified timeout period, the source changes the PREF field in the packet header to indicate that weak channels have been accepted, as these may be the only links over which the packet can be propagated. When a failed link is detected in the network, route error packets are sent and another search process is initiated. The source also sends an erase message to notify all the nodes about the broken link. 2.5.2 Power Aware Routing In this protocol, power-aware metrics are used for determining routes in MANETs. It has been shown that using these metrics in a shortest-cost routing algorithm reduces the cost/packet of routing packets by 5 -30 percent over shortest-hop routing (this cost reduction is on top of a 40-70 percent reduction in energy consumption over the MAC layer protocol used). Furthermore, using these new metrics ensures that mean time to node failure is increased significantly, while packet delays do not increase. A recent work concentrates on selecting a route based the traffic and congestion characteristics in the network. 2.5.3 Associativity-Based Routing This is a totally different approach in mobile routing. The Associativity-Based Routing (ABR) protocol is free from loops, deadlock, and duplicate packets. A fundamental objective of ABR is to derive long-lived routes for ad hoc networks. In ABR, a route is selected based on a metric that is known as the degree of association stability. Each node periodically generates a beacon to signify its existence. When received by neighboring nodes, this beacon causes their associativity tables to be updated. For each beacon received, the associativity tick of the current node with respect to the beaconing node is incremented. A high (low) degree of association stability may indicate a low (high) state of the node mobility. Associativity ticks are reset when the neighbors of a node or the node itself move out of the proximity. The three phases of ABR are: • Route discovery; • Route reconstruction (RRC); • Route deletion. K. V. Pradeep

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The route discovery phase is accomplished by a broadcast query and await-reply (BQ-REPLY) cycle. A node desiring a route broadcasts a BQ message in search of MHs that have a route to the destination. All nodes receiving the query (that are not the destination) append their addresses and their associativity ticks with their neighbors along with QoS information to the query packet. A successor node erases its upstream node neighbors' associativity tick entries and retains only the entry concerned with itself and its upstream node. In this way, each resultant packet arriving at the destination contains the associativity ticks of the nodes along the route to the destination. If multiple paths have the same overall degree of association stability, the route with the minimum number of hops is selected. The destination then sends a REPLY packet back to the source along this path. Nodes propagating the REPLY mark their routes as valid. All other routes remain inactive, and the possibility of duplicate packets arriving at the destination is avoided. RRC may consist of partial route discovery, invalid route erasure, valid route updates, and new route discovery, depending on which node(s) along the route move. Movement by the source results in a new BQ-REPLY process. When the destination moves, the immediate upstream node erases its route and determines if the node is still reachable by a localized query (LW]) process, where H refers to the hop count from the upstream node to the destination. If the destination receives the LQ packet, it REPLYs with the best partial route; otherwise, the initiating node times out and the process backtracks to the next upstream node. Here, a RN message is sent to the next upstream node to erase the invalid route and inform this node that it should invoke the LQ[H] process. If this process results in backtracking more than halfway to the source, the LQ process is discontinued and a new BQ process is initiated at the source. 2.5.4 QoS Routing All the routing protocols discussed so far have been proposed either for routing messages along the shortest available path or within some systemlevel requirement. Routing applications using these paths may not be adequate for applications which require QoS (e.g., real-time applications). In this section we overview some routing schemes that can support QoS in MANETs. Figure 2.19 illustrates an example where nodes are labeled as A, B, C, ..., J. The numbers along each edge represent the available bandwidths of the wireless links If we want to find a route from a source node A to a destination K. V. Pradeep

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Routing in Ad-Hoc Networks node G, conventional routing using shortest path (in terms of number of hops) as a metric, selects the route A-B-H-G. However, the QoS-based route selection process from node A to node G with a minimum bandwidth of 4 would use A-B-C-D-E-F-G as one possible path over the shortest path route A-B-H-G.

The QoS-aware path is determined within the constraints of bandwidth, minimal search, distance, and traffic conditions. To date, only a few QoS-aware routing protocols have been proposed for MANETs and we review the most prominent ones in the following sections. 2.5.4.1 Core Extraction Distributed Ad Hoc Routing The Core Extraction Distributed Ad Hoc Routing (CEDAR) algorithm is a partitioning protocol proposed as a QoS routing scheme for small to medium size MANETs consisting of tens to hundreds of nodes. It dynamically establishes the core of the network, and then incrementally propagates the link states of stable high-bandwidth links to the core nodes. CEDAR has three key components: • Core Extraction: A set of nodes is elected to form the core that maintains the local topology of the nodes in its domain, and also approximating a minimum dominating set of the MANET. • Link State Propagation: QoS routing in CEDAR is achieved by propagating the bandwidth availability information of stable links to all core nodes. The basic idea is that the information about stable highbandwidth links can be made known to the nodes far away in the network, while information about the dynamic or low bandwidth links remains within the local area. • Route Computation: Route computation first establishes a core path from the domain of the source to the domain of the destination. Using K. V. Pradeep

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Routing in Ad-Hoc Networks the directional information provided by the core path, CEDAR iteratively tries to find a partial route from the source to the domain of the furthest possible node in the core path, satisfying the requested bandwidth. This node then becomes the source of the next iteration. In the CEDAR approach, the core provides an efficient and lowoverhead infrastructure to perform routing, while the state propagation mechanism ensures availability of link-state information at the core nodes without incurring high overheads. 2.5.4.2 Incorporating QoS in Flooding-Based Route Discovery A ticket-based probing algorithm with imprecise state model has been proposed in [Chen1998] for discovering a QoS-aware routing path, by issuing a number of logical tickets to limit the amount of flooding (routing) messages. When a probing message arrives at a node, it may be split into multiple probes and forwarded to different next-hops with each child probe containing a subset of the tickets from their parents. When one or more probe(s) arrive(s) at the destination, the hop-by-hop path known and delay/bandwidth information can be used to perform resource reservation for the QoS-satisfying path. In wired networks, a probability distribution can be calculated for a path based on delay and bandwidth information. In a MANET, however, building such a probability distribution is not suitable because wireless links are subject to breakage and state information is inaccurate. Therefore, a simple imprecise model has been proposed using the history and current (estimated) delay variations which is represented as a range of [delay - 8, delay + 4. To adapt to the dynamic topology of MANETs, this algorithm allows different level of route redundancy. When a node detects a broken path, it notifies the source node which will reroute the connection through a new feasible path, and notifies the nodes along the old path to release the corresponding resources. Unlike the re-routing technique, the path-repairing technique does not find a completely new path. Instead, it tries to repair the path using local reconstructions. Another approach for integrating QoS in the looding-based route discovery process has been proposed. This proposed positional attributebased next-hop determination approach (PANDA) discriminates the next hop nodes based on their location or capabilities. When a route request is broadcast, instead of using a random broadcast delay, the receivers opt for a delay proportional to their abilities in meeting the QoS requirements of the path. The decisions at the receiver side are made on the basis of a predefined set of rules. Thus, the end-to-end path will be able to satisfy the QoS constraints as long as it is intact. A broken path will initiate the QoS-aware route discovery process. K. V. Pradeep

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2.5.4.3 QoS Support Using Bandwidth Calculations An available bandwidth calculation algorithm for MANETs where time division multiple access (TDMA) is employed for communications is proposed which involves end-to-end bandwidth calculation and allocation and, the source node can determine the resource availability for supporting the required QoS. This approach is particularly useful in call admission control. In wired networks, the path bandwidth is the minimum available bandwidth of the links along the path. In time-slotted ad hoc networks, however, bandwidth calculation is much harder. In general, we not only need to know the free slots on the links along the path, but also need to determine how to assign the free slots at each hop. Figure 2.20 illustrates a simple example, where time slots 1, 2, and 3 are free between nodes A and B, and slots 2, 3, and 4 are free between nodes B and C. Assume node A wants to send some data to node C. Note that there will be collisions at node B if node A tries to use all three slots 1, 2, and 3 to send data to node B while node B is using 2 and 3 to send data to node C. Thus, we have to one or both slots somehow divide the common free slots 2 and 3 between the two links, namely, from node A to node B, and from node B to node C

In TDMA systems, time is divided in slots which, in turn, are grouped into frames. Each frame contains two phases: control and data phases. During the control phase, each node takes turns to broadcast its information to all of its neighbors in a predefined slot. Hence, at the end of the control phase, each node has learned the free slots between itself and its neighbors. Based on this information, bandwidth calculation and assignment can be carried out in a distributed manner. Determining slot assignments while searching for the available bandwidth along the path is a NP-complete problem. Thus, a heuristic approach to tackle this issue has been proposed. An on-demand QoS routing protocol using AODV has been designed for TDMA-based MANETs in [Zhu2002]. In this approach, a QoS-aware route reserves bandwidth from source to destination. In the route discovery procedure of AODV, a distributed algorithm is used to calculate the K. V. Pradeep

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Routing in Ad-Hoc Networks available bandwidth on a hop-by-hop basis. Route request messages with inadequate bandwidth are dropped by intermediate nodes. Only the destination node can reply to a route request message that has come along a path with sufficient bandwidth. The protocol can handle limited mobility by repairing broken paths. This approach is best applicable for small size networks or for short routes. 2.5.4.4 Multi-Path QoS Routing A multi-path QoS routing protocol has been introduced in which is suitable for ad hoc networks with very limited bandwidth for each path, unlike other existing protocols for MANETs, which try to find a single path between the source and the destination, this algorithm searches for multiple paths for the QoS route. This protocol also adopts the idea of ticketbased probing scheme discussed earlier. Another rational for using multipath routing is to enhance the routing resiliency by finding node/edge disjoint paths when link and/or node fail. Another approach is to use the extension of AODV to determine a backup source-destination routing path that could be used if the path gets disconnected frequently due to mobility or changing link signal quality. An analytical model has been developed to justify having a backup path which can be easily piggybacked in data packets. Steps for immediate repairs of broken backup routes have also been suggested and extensive simulations have been done to validate the effectiveness of this scheme. Homework Questions/Simulation Results Q. 1. Ad hoc networks are special kinds of wireless network that does not have any underlying infrastructure. But, such networks are becoming increasingly important for both defense and civilian applications. Assuming a 60 X 60 grid connected ad hoc network is given to you, and the address of each node is given by (i,j) with 0=
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